The present invention relates generally to Global Positioning System (GPS), and more specifically, to improved signal detection acquisition time and low-level signal detection for GPS receivers.
According to the Federal Communications Commission (FCC) cellular radiotelephone calls must be geographically locatable. This capability is desirable for emergency services systems such as E911. The FCC requires stringent accuracy and availability performance objectives and demands that cellular radiotelephones be locatable within 50 meters 67% of the time, and 150 meters 95% of the time for handset based solutions, and within 100 meters 67% of the time and 300 meters 95% of the time for network based solutions. Even this relaxed threshold for network-based solutions has been difficult to achieve using traditional TOA/TDOA (Time Of Arrival/Time Difference Of Arrival) infrastructure technology.
In order to include Global Positioning System (GPS) in wireless portable devices such as cellular radiotelephones; performance needs to be improved in several areas including weak signal detection, acquisition time and energy use for operating power. Regarding weak signal detection, users of cellular radiotelephones have become accustomed to making calls indoors, and traditional processing of GPS signals will not accommodate the attenuation caused by most buildings. Since these GPS receivers capture signals from satellites at quite an extraordinary distance, any objects in the direct line of sight between the GPS receiver and the satellites often cause a malfunction because the signal transmitted by the satellites is attenuated by the interfering object making it difficult for the GPS receiver to receive them. Trees, buildings, and other high-profile objects can cause line of sight interference resulting in the problem of weak or low signal detection.
Regarding accuracy, differential GPS approaches may work but are complex and costly. Moreover they don""t address the weak signal problem. This major problem with traditional GPS signal processing techniques involves bandwidth and signal power. The GPS satellites transmit a very weak signal; guaranteed signal levels are only xe2x88x92130 dBm on the surface of the earth. Actual signals as measured on the earth""s surface show signal levels of about xe2x88x92125 dBm. The acquisition threshold of current automotive and consumer grade handheld GPS receivers is on the order of xe2x88x92137 dBm, thus the link margin for signal acquisition is only about 7 to 12 dB.
A sequential detection algorithm is used by almost every GPS receiver on the market in order to acquire the spread spectrum signals of the GPS satellites. One can extend the acquisition threshold to lower levels by lengthening the pre-detection integration (PDI) interval at the expense of acquisition time. Even so, there is a maximum PDI of about 12 milliseconds (83 Hz bandwidth) beyond which the sequential detection process breaks down. This is because the GPS signal structure includes BPSK modulated fifty bits-per-second (50 BPS) navigation data transmitted on top of the 1.023 MHz spreading code that ultimately limits how long one can coherently integrate in order to increase the SNR. Beyond 20 ms (one data bit time), the data bit transitions cause the integration sum to be reduced or sum to zero, depending on the phase relationship of the integration period relative to the data bit transition.
Another problem is that contemporary GPS receivers are often embedded, within portable devices where energy is derived from a battery. These portable devices include devices such as cellular radiotelephones, PDAs (Personal Digital Assistants), portable computers, surveying devices and other devices that make use of information provided by a GPS receiver. When these GPS receivers operate, they consume a substantial amount of energy, which depletes energy from the battery that could be made use of by the co-embedded functions. If GPS correlation can be done faster, battery energy can be conserved because the GPS receiver can be turned off when correlation is achieved. Prior art schemes have inadequately addressed energy conservation.
Yet another problem involves inaccurate internal reference oscillators in GPS receivers, which increases correlation times. Oscillator inaccuracy increases satellite signal acquisition times by increasing the Doppler search space. Others have therefore endeavored to provide accurate reference oscillators, especially low cost oscillators, for GPS receivers. One solution has used the stored oscillator temperature characteristic data to match reference and GPS oscillator frequencies. Another solution uses stored temperature frequency offset data for subsequent GPS signal acquisition. However, these solutions are still not accurate enough and require additional memory. Still another solution uses a precision carrier frequency signal from a terrestrial network to generate a reference signal for calibrating a local oscillator used by a GPS receiver. And another solution freezes the oscillator correction signal when the GPS receiver makes position determinations. Still another solution uses a precision carrier frequency signal from a terrestrial network to generate a reference signal for controlling a local oscillator signal to a GPS receiver. However, these solutions require added hardware systems and are computationally complex.
What is needed is an improved GPS signal acquisition method and system that can operate with weaker signals and lock onto satellite signals faster than prior art schemes, particularly for E911 calls. Moreover, it would be of benefit to provide a simple method to correct for oscillator error in a GPS system.